Need help on a 65/66 Shelby Mustang question

[SS427]

I have a technical question and am hoping someone can help me. I have a very old 65/66 Mustang fender that I believe to be an original Shelby fender that I got back in the 80's though a couple of things throw me.

It is in original paint but has grey primer underneath. The white measures 3 mils while the stripes measure 6 mils. I honestly cannot tell if the GT350 decals are vinyl or paint. If I lightly sand the stripe there is white underneath but it is not the white of the body paint as there is still a ridge on the edge. They appear to be the 66 style based on the blue stripe being before and after the GT350 unlike the 65 where the 350 bordered the door jamb on the driver's side from what I have researched.

I found dates in three places. The fender apron is dated 6 4 3, the headlight bucket is dated 2 12 (or 19) 3 though it is hard to read (see photos) and the inner brace near the rear of the fender is dated 3 22 D3. What is odd is that the D stands for Dearborn if I am correct and from what I know, no 65 or 66 Shelbys were made at Dearborn if that is also correct. I am just confused as this fender looks so original. Any input you can provide would be greatly appreciated. Rick